Nonsense suppressors in the eucaryote Saccharomyces cerevisiae have been studied. Amber suppressors previously grouped together as class 4 (Liebman et al. Genetics, 1976, 82: 251) on the basis of their common phenotypes, have now been placed into allelic groups. A suppressor from one group has been genetically mapped and it represents a previously unmapped suppressor locus. Antisuppressors which reduce the efficiency of the two "omnipotent" suppressors, sup35 and sup45, have been isolated. These antisuppressors are not at the sup35 or sup45 loci. Detailed genetic studies of three such antisuppressor mutations have been undertaken. It is hoped that these mutants may represent additional genes involved in protein synthesis such as tRNA maturation or modification enzymes, ribosomal proteins or termination factors.